The present invention relates to a non-sequential raster scanning method, and more particularly to a method useable with a cathode ray tube (CRT) to maintain a more uniform screen light intensity during the scanning function.
A cathode ray tube (CRT) consists of an evacuated glass tube having a flat screen area treated by a uniform layer of unexcited phosphorous. An electron gun producing a deflectable electron beam shoots electrons toward the phosphorous treated screen. When the phosphorous atoms which coat the interior surface of the screen are struck by the electron beam, they become excited and emit visible light glowing on the screen surface. If the electron beam is continuously scanned in a linear fashion upon the CRT screen, the glowing phosphorous atoms produce a raster line visible on the CRT screen surface. A vertical series of adjacent horizontal raster lines appear as a rectangular glowing raster scan of the generally circular CRT screen.
The ability of the CRT raster scan to glow to a level of brightness depends upon the size of the raster scan area.
When a limited CRT screen area is repeatedly scanned by the electron beam heat buildup or thermal changes affect the phosphorous coating the interior surface of the CRT. Phosphorous experiencing thermal changes emit light inefficiently.
For example, when a CRT is used in a film-to-video transfer apparatus like a telecine, the size of raster scan on the CRT screen surface adjusts to compensate for velocity changes of the film being transferred past the CRT. Increased film scanning speed generally produces a decrease in the raster scan size and a detrimental change in the efficiency of the light producing phosphorous.
Normal raster scan is performed during the transfer of a film image to a video signal. If raster scan is interrupted, to perform film weave correction as disclosed in U.S. patent application Ser. No. 07/502,472, filed Mar. 30, 1990 entitled "Real Time Registration Weave Correction System", the electron beam may return to a CRT screen area previously illuminated or a screen area where adjacent scanning has occurred. Adjacent raster lines scanned numerous times over a short period of time produces heat buildup and thermal changes which reduce the efficiency of the phosphorous. The decrease in efficiency of the phosphorous causes the phosphorus to glow more dimly affecting the light level of the transferred video image.
Continued raster size decrease or long-term adjacent energizing of a small, localized screen area, also burns a patch into the phosphorous. This patch is an area of decreased phosphorus efficiency and appears more dim than the rest of the raster scan.
Another scanning problem occurs during scan tracking in the transfer of film to video using a telecine. Phosphorous grain size becomes very large exposing blemishes in the transferred film being scanned producing a shadowy, web-like haze over the resultant transferred video picture.